This invention relates generally to digital electronic systems, in particular to waveform restoration systems used therein. Digital systems require that signals be in either of two states: "high" (voltage exceeding a specified value) or "low" (voltage less than a specified value) except during very fast transitions between these states. Any voltage between these states produces indeterminate results in the system, and therefore must be eliminated.
When digital information is transmitted between digital systems or retrieved from magnetic recording media, low frequency interference frequently causes the signal to spend time in this indeterminate state, causing erroneous digital information to be generated in the system. One cause of this low frequency contamination is 60 hertz power line magnetic fields or ground currents.
One prior art circuit used to eliminate low frequency interference is a differentiating network, which passes only the high frequency components of the applied signal. This device is unsatisfactory for use in digital circuits, since the output signal always returns to a "low" within one time constant and hense can not respond properly to digital signals which remain "high" for long periods of time. (The time constant of an R-C differentiating network is equal to the value of the resistor in ohms times the value of the capacitor in farads, with the result in seconds.)
A second prior art circuit is a (negative) feedback differentiator, which bridges a differentiating network across a linear inverting amplifier. The negative feedback provided by the linear inverting amplifier results in a more perfect differentiated signal at the output terminal. This negative feedback also causes the output to return to a fixed voltage, usually in the center of the active region, an undesired condition as described in the first paragraph of this section. Although these systems bear some similarity to the present invention, they are not used for the same purpose, because of the aforementioned limitations.
Because of the need for optimum waveform in digital systems and the many causes of interference and waveform distortion in recording and communication equipment, complex encoding and decoding schemes are presently used for this purpose. These schemes severely limit the rate at which digital information can be transferred in these systems.